Information recording disc having light absorbing cellulose nitrate coating

ABSTRACT

An information recording disc comprising a generally circular substrate having a polished major surface, a light-reflective coating formed on the polished major surface of the substrate, and a light-absorptive coating formed on the surface of the light-reflective coating and formed with information tracks each consisting of a succession of spaced pits in the light-absorptive coating, the light-absorptive coating being composed of a mixture of cellulose nitrate and a dye sensitizer exhibiting a noticeable light absorptivity in response to light with a predetermined wavelength, the cellulose nitrate being prepared from a starting cellulose containing at least about 97% of alpha-cellulose. If desired, the light-reflective coating may be dispensed with so that the light-absorptive information recording coating is formed directly on the polished surface of the substrate.

FIELD OF THE INVENTION

The present invention relates to an information recording disc for useas, for example, a video disc or a digital audio disc and further to aprocess of producing such an information recording disc.

BACKGROUND OF THE INVENTION

An information recording disc such as a digital audio and video disc hasusually been produced by a photoresist etching process. At an initialstage of the photoresist etching process, a disc-shaped substrate of,for example, glass having a polished flat major surface is rinsed andthereafter cooled at a predetermined temperature. About an hour isconsumed for these steps. The substrate of glass is then coated with athin layer of photoresist and the resultant laminar structure isprebaked and cooled at a predetermined temperature. Another hour isrequired for these steps. Upon completion of the cooling step, thelight-absorptive photoresist coating of the laminar structure isirradiated with a beam of, for example, laser light which is modulatedin intensity in accordance with the signals to be recorded on the disc.About one to two hours are required for this step. The laminar structurehaving the light-absorptive coating thus processed is then developed,rinsed in water, dried, afterbaked and thereupon further cooled, aboutan hour being required for these steps. Spiral or coaxial informationtracks each consisting of a succession of spaced pits arranged inpatterns representative of the pieces of information to be reproducedare formed on the surface of the light-absorptive coating of thephotoresist thus processed. A light-reflective coating of metal is thenapplied to the surface of the light-absorptive coating, about forty fiveminutes being required for this step. The disc blank now having thelight-absorptive and light-reflective coatings on the substrate of glassis transferred to an inspection stage where the disc blank is playedback on an experimental basis so as to check for any flaws such asdropouts of signal level. The inspection stage requires a period ofabout one to two hours. If found acceptable at this stage, the discblank is transferred to a stage where a stamper is produced are replicasproduced in quantity from the stamper.

A total of four to five hours are thus required from the step ofpreparing an initial substrate to the step of inspecting the substratecoated with the light-reflective metal coating. This means that aninformation recording disc can not be rejected or determined as beingacceptable earlier than about four to five hours after the originalsubstrate is coated with photoresist. If the recording disc producedturns out not to be acceptable at the inspection stage, viz., four tofive hours after the original substrate has been coated with thephotoresist, another disc must be produced by repeating all the stepsabove described. This is manifestly detrimental to the economy andefficiency of producing an information recording disc. Such a problemresults from the fact that the signals recorded on the disc can not bereproduced or monitored immediately after the signals are recorded orwhile the signals are being recorded on the photoresist coating on thesubstrate.

The disc produced may also be rejected if the disc, on which the signalsto be recorded have been properly recorded, is developed for an undulyextended or short period of time. Because, furthermore, of the fact thatthe photoresist is sensitive not only to a beam of laser light but tonatural light, the laminar structure having the coating of thephotoresist can not be processed in natural light. This is alsodetrimental to the efficiency of production of an information recordingdisc. All these problems encountered in producing an informationrecording disc basically result from the fact that a photoresist is usedto form the light-absorptive, information recording coating of the disc.

The signals recorded on the light-absorptive coating on a substrate canbe monitored during recording of the signals in a process taught in, forexample, U.S. Pat. No. 4,097,895. In the prior-art process thereindisclosed, a light-absorptive information recording coating of anorganic dye (fluorescein) which is absorptive exclusively to light of apredetermined wavelength is formed on a light-reflective coating ofaluminum preliminarily formed on a disc-shaped substrate of glass. Whenthe recording coating of the disc thus composed is irradiated with abeam from an argon laser, the dye exhibits a noticeable absorptivity inresponse to a laser beam of the particular wavelength and causesablation of the coating and produces a pit at the irradiated spot. Theinformation recording coating thus formed with pits is then irradiatedwith a beam from an argon laser with a power selected to be less thanthe power of the initial laser radiation. The secondarily radiated laserbeam is insufficient in power to be capable of causing ablation of thelight-absorptive coating of the dye so that the beam is reflected atthose spots where the reflective coating underlying the absorptivecoating is exposed through the pits formed by the initial laserradiation. At the undisturbed areas, viz., those areas of thelight-absorptive coating where the layer of the dye has not beenirradiated with the laser beam, the secondarily radiated laser beam isabsorbed and is as a consequence not reflected (or is reflected only toa negligible degree). The presence or absence of a pit and accordinglythe signal recorded on the disc can be detected through detection of thedifference between the luminous energy of the laser beam incident on theexposed light-reflective coating and that of the laser beam incident onthe light-sensitive information recording coating.

The process taught in U.S. Pat. No. 4,097,895 is advantageous in thatthe signals recorded on an information recording disc composed of asubstrate and light-reflective and light-absorptive coatings can bemonitored during recording thereof, in that a wet developing step is notneeded, and in that the disc with the light-absorptive coating can beprocessed in the light since the dye forming the coating shows anoticeable absorptivity exclusively in response to a beam of light of apredetermined wavelength. These advantages are however offset by theprolonged period of time required for causing the ablation of the dyeand the difficulty encountered in forming clear-cut pits in thelight-absorptive recording coating. The failure to achieve clear-cutpits in the light-absorptive coating results in a degradedsignal-to-noise ratio of the resultant information recording disc.

An improved version of the process taught in the above named patent isdisclosed in, for example, Japanese Provisional Patent Publication No.55-87595 and the article titled "Real-Time Laser Recording Using DyeVaporization Recording Substance" in Procedings of Institute ofElectronics and Communications Engineers of Japan (CMP 79-59) issued onNov. 22, 1979, Tokyo. In the process disclosed therein, a laser beam ofa predetermined wavelength is focused on an information recording discconsisting of a substrate of acryl or a transparent coating of polyesteror polyethylene and a coating of the mixture of a dye sensitizer andcellulose nitrate dissolved in a solvent of the ketone family. The dyesensitizer may be Ethyl Red, Methylene Blue or Brilliant Green and thecellulose nitrate used has a degree of polymerization approximating 80.The wavelength of the laser beam to be used with such an informationrecording disc is selected so that the dye sensitizer exhibits anexceptional absorptivity in response to the particular wavelength.

The use of cellulose nitrate in addition to a dye in forming alight-absorptive information recording coating permits the informationrecording coating to be sublimated at an increased rate due to theself-oxidation effect of the cellulose nitrate. The informationrecording disc produced in such a process is therefore advantageous inthat the ablation of the light-absorptive coating proceeds in a reducedperiod of time, in that a laser beam of a reduced power can be used informing pits in the recording coating or, in other words, signals can berecorded on the coating at a reduced temperature and in that the pitsformed in the light-absorptive coating have configurations clearer thanthose of the pits formed in an information recording disc produced inthe process originally proposed in U. S. Pat. No. 4,097,895. Problemsare, however, still encountered in the advanced process in that theclearness of the configurations of the pits is inferior to that of thepits formed in a light-absorptive coating of photoresist and in that thereaction of the dye and cellulose nitrate with the laser radiationproduces considerable quantities of chemical residua on the surface ofthe light-absorptive coating. The chemical residua thus deposited on thelight-absorptive coating impair the signal-to-noise ratio of theresultant information recording disc. By reason of these problems, theprocess still remains at an experimental stage of research anddevelopment and is not acceptable as means to supercede the conventionalphotoresist etching process when put into practice on a commercialbasis.

It is, accordingly, an important object of the present invention toprovide an information recording disc which will permit monitoring ofthe signals during or immediately upon completion of the recording ofthe signals on a light-absorptive information recording coating.

It is another important object of the present invention to provide aninformation recording disc which can can be produced without havingrecourse to the wet developing step which is required in a conventionalphotoresist etching process.

It is still another important object of the present invention to providean information recording disc having an excellent signal-to-noise ratio.

It is still another important object of the present invention to providean information recording disc having a light-absorptive informationrecording coating formed with satisfactorily clear-cut pits.

It is still another important object of the present invention to providean information recording disc which can be produced with practicallynegligible quantities of chemical residua produced on the surface of thelight-absorptive coating by irradiation of the information recordingcoating of the disc with a recording beam of light.

It is, yet, another important object of the present invention to providea process of producing an information recording disc of the abovedescribed nature.

SUMMARY OF THE INVENTION

In accordance with one outstanding aspect of the present invention,there is provided an information recording disc comprising a generallycircular substrate having a polished major surface, and alight-absorptive coating formed on the polished major surface of thesubstrate and formed with information tracks each consisting of asuccession of spaced pits in the light-absorptive coating, thelight-absorptive coating being composed of a mixture of cellulosenitrate and a dye sensitizer exhibiting a noticeable light absorptivityin response to light with a predetermined wavelength, the cellulosenitrate being prepared from a starting cellulose containing at leastabout 97% of alpha-cellulose. As an alternative, an informationrecording disc according to the present invention may comprise agenerally circular substrate having a polished major surface, alight-reflective coating formed on the polished major surface of thesubstrate, and a light-absorptive coating formed on the surface of thelight-reflective coating and formed with information tracks eachconsisting of a succession of spaced pits in the light-absorptivecoating, the light-absorptive coating being composed of a mixture ofcellulose nitrate and a dye sensitizer exhibiting a noticeable lightabsorptivity in response to light with a predetermined wavelength, thecellulose nitrate being prepared from a starting cellulose containing atleast about 97% of alpha-cellulose.

In accordance with another outstanding aspect of the present invention,there is provided a process of producing an information recording disc,comprising the steps of preparing a generally circular substrate havinga polished major surface, forming a light-absorptive coating on thepolished major surface of the substrate, the light-absorptive coatingbeing composed of a mixture of cellulose nitrate and a dye sensitizerexhibiting a noticeable light absorptivity in response to light with apredetermined wavelength, the cellulose nitrate being prepared from astarting cellulose containing at least about 97% of alpha-cellulose, andirradiating the surface of the light-absorptive coating with a beam oflight having the aforesaid wavelength for forming information trackseach consisting of a succession of spaced pits in the light-absorptivecoating. As an alternative, a process according to the present inventionmay comprise the steps of preparing a generally circular substratehaving a polished major surface, forming a light-reflective coating onthe polished major surface of the substrate, forming a light-absorptivecoating on the surface of the light-absorptive coating, thelight-absorptive coating being composed of a mixture of cellulosenitrate and a dye sensitizer exhibiting a noticeable light absorptivityin response to light with a predetermined wavelength, the cellulosenitrate being prepared from a starting cellulose containing at leastabout 97% of alpha-cellulose, and irradiating the surface of thelight-absorptive coating with a beam of light having the aforesaidwavelength for forming information tracks each consisting of asuccession of spaced pits in the light-absorptive coating.

BRIEF DESCRIPTION OF THE DRAWING

The features and advantages of a process according to the presentinvention and an information recording disc produced by such a processwill be more clearly appreciated from the following description taken inconjunction with the accompanying drawing in which:

FIG. 1 is a fragmentary cross sectional view showing an informationrecording disc embodying the present invention; and

FIG. 2 is a graph showing examples of the relationship between thepercentage transmission and the wavelength of the layers composed ofcellulose nitrate and dye sensitizer in various proportions.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 of the drawing, an information recording discembodying the present invention, as designated in its entirety byreference numeral 1, comprises a generally circular substrate 2 whichhas been preliminarily processed to have a flat polished major surface.The information recording disc 1 further comprises a light-reflectivecoating 3 of metal such as, for example, silver or aluminum formed onthe polished major surface of the substrate 2 and a light-absorptivecoating 4 consisting of a mixture of cellulose nitrate and a dyesensitizer and formed on the light-reflective coating 3. Thelight-absorptive coating 4 is formed with spiral or coaxial informationtracks of spaced pits 5 which are arranged in patterns representative ofthe signals to be reproduced. The information recording disc 1 hereinshown is assumed, by way of example, to be for use as a master matrixfrom which a number of replicas are to be produced and, thus, thesubstrate 2 is made of transparent glass. If the disc 1 is to be used asa final record, then the substrate 2 may be formed of a synthetic resinsuch as polyester or acryl.

At an incipient stage of the process of producing the informationrecording disc 1, the glass substrate 2 is rinsed, cooled and thereafterformed with the light-reflective coating 3. The light-reflective coating3 is vacuum evaporated, sputtered or otherwise applied onto the polishedmajor surface of the substrate 2 to a predetermined thickness which mayrange from about 200 angstrom units to about 400 angstrom units. If thethickness of the coating 3 is less than the lower limit of this range, asufficient amount of luminous energy could not be obtained when a signalpick-up laser beam is focused on the light-reflective coating 3 duringmonitoring of the signals recorded on the disc at a later stage. On theother hand, a thickness of the light-reflective coating 3 exceeding theupper limit of the range above specified would result in a mere waste ofmaterial although such a thickness would cause no problem in respect ofthe function of the resultant information recording disc. Experimentshave proved that, when the light-reflective coating 3 is formed to thethickness of about 200 angstrom units, the reflectance of thelight-reflective coating 3 to a beam of helium-neon laser used as asignal pick-up medium is within the range of between about 80% and about85%.

Purified cotton linters containing 97% or more of alpha-cellulose arepreferable as the starting cellulose to prepare the cellulose nitrate inthe light-absorptive coating 4. If the proportion of the alpha-cellulosein the cotton linters is less than 97%, the result is that considerablequantities of chemical residua are produced and deposited on the surfaceof the light-absorptive information recording coating 4 as a result ofthe reaction of the mixture of the cellulose nitrate and dye sensitizerwith laser radiation and accordingly that a satisfactory signal-to-noiseratio could not be achieved of the resultant information recording disc.The number average degree of polymerization of the cellulose nitratecontaining the alpha-cellulose in a proportion within the range abovespecified is preferably within the range of from about 80 to about 95(which corresponds to RS 1/2 in JIS=Japanese Industrial Standard). Alesser degree of polymerization of the cellulose nitrate would make itdifficult to form satisfactorily clear-cut pits to be formed in thelight-absorptive coating 4, while a higher degree of polymerizationwould result in an excessive viscosity of the cellulose nitrate and makeit difficult to achieve uniformity of thickness of the light-absorptivecoating 4. The cotton linters to produce such cellulose nitrate isnitrated with, for example, a mixture of nitric acid, sulfuric acid andwater as usual. In this instance, the degree of nitration of thestarting cellulose is preferably selected to be higher than about 13%or, more preferably, than 13.32%. If the degree of nitration of thecotton linters is lower than about 13%, pits could not be formed in thelight-absorptive coating 4 unless a signal recording beam of aconsiderably increased power is used. The use of an excessively powerfullaser beam would result in deterioration in the configurations of thepits 5 formed in the light-absorptive coating 4. The cellulose nitrateprepared as above described is dissolved in a suitable organic solventsuch as xylene or a mixed solution of xylene and ethyl cellosolveacetate and the resultant solution is passed through a filter of apredetermined mesh size of, for example, 0.2 micrometer.

The dye sensitizer in the light-absorptive coating 4 isW-ethyl-N-oxyethylaniline. The dye sensitizer is also dissolved in asuitable organic solvent such as xylene or a mixed solution of xyleneand ethyl cellosolve acetate and the resultant solution is passedthrough a filter of, for example, the above specified mesh size.

Thereupon, the solution containing the cellulose nitrate and thesolution containing the dye sensitizer are mixed together in aproportion selected so that the cellulose nitrate and dye sensitizer arecontained in a predetermined weight ratio in the resultant mixture. Asuitable organic solvent is added to the mixture in a volume selected sothat the solvent is in the ratio of about 46 milliliters to 1 gram ofthe mixture of the cellulose nitrate and dye sensitizer. The resultantmixed solution is heated to about 40° C. and is shaked for about fourhours. The mixed solution is then passed through a filter of, forexample, the above specified mesh size and is thereafter applied to thelight-reflective coating 3 on the substrate 2.

When the ratio by weight between the cellulose nitrate and dyesensitizer forming the light-absorptive coating 4 is varied, the percenttransmission of light through the coating 4 varies in terms ofwavelength of light as indicated in the graph of FIG. 2. In FIG. 2,plots a, b, c and d show the variations of the percent transmission asobserved when the ratio by weight between the cellulose nitrate and thedye sensitizer is selected at 95.0:5.0, 90.0:10.0, 86.5:13.5 and80.0:20.0, respectively. From these plots a, b, c and d it will be seenthat the layer of the cellulose nitrate and dye sensitizer exhibits anoticeable absorptivity in response to light of a wavelength within therange of between about 440 nanometers and about 530 nanometers in everycase and that the larger the proportion of the dye sensitizer the higherthe percent transmission of light. When, for example, the weight ratiobetween the cellulose nitrate and dye sensitizer is selected at86.5:13.5 as indicated by plot c, the kinematic viscosity of the mixtureof the cellulose nitrate and dye sensitizer becomes 3.6 centistokes atthe temperature of 25° C. and the layer of the cellulose nitrate and dyesensitizer exhibits the percent transmission of about 35% andaccordingly the percent absorption of about 65% when irradiated with abeam of argon laser having the wavelength of 456.1 nanometers

The mixed solution to form the light-absorptive coating 4 is applied tothe surface of the light-reflective coating 3 by a spin coating process.For this purpose, 10 milliliters of the mixed solution is dropwiseapplied to the surface of the light-reflective coating 3, whereupon thelaminar structure of the substrate 2 and light-reflective coating 3 isdriven for rotation about the center axis thereof at the speed of 250rpm for the period of 11 seconds and thereafter further at the speed of600 rpm for the period of 61 seconds. By these steps in which thelaminar structure is driven for rotation first at a relatively low speedand thereafter at a higher speed, the mixed solution applied to thesurface of the light-reflective coating 3 forms a layer having a uniformthickness of about 2500 angstrom units as the light-absorptiveinformation recording coating 4.

The laminar structure thus composed of the substrate 2, light-reflectivecoating 3 and light-absorptive coating 4 is then baked at thetemperature of about 80° C. for the period of about 20 minutes so thatthe organic solvent is removed from the light-absorptive coating 4.Removing the solvent from the light-absorptive coating 4 is importantsince, if the organic solvent remains in the light-absorptive coating 4,then the information recording laser beam to form the pits 5 in thecoating 4 in a later step must be emitted with a power required not onlyfor causing ablation of the light-absorptive coating 4 but also forevaporating the solvent from the coating 4. Irradiation of thelight-absorptive coating 4 with a beam of light of an insufficient powerwould lead to failure to form acceptable pits in the light-absorptivecoating 4 and further to creation of chemical residua on the surface ofthe coating 4. Depending upon the type of the dye sensitizer used,furthermore, evaporation of the organic solvent from thelight-absorptive coating 4 might cause irregularity of thickness of thecoating 4. The dye sensitizer consisting of N-ethyl-N-oxyethylaniline isfree from such a problem. After the organic solvent has been completelyremoved from the light-absorptive coating 4, the laminar structure ofthe substrate 2 and the light-absorptive and light-reflective coatings 3and 4 is cooled for a certain period of time.

The step thus forming the light-absorptive coating 4 on thelight-reflective coating 3 is followed by a mastering step to form theinformation tracks of the spaced pits 5 in the light-absorptive coating4. In this mastering step, the laminar structure is driven for rotationat a predetermined circumferential speed of, for example, 1.25 metersper seconds and is concurrently irradiated with a beam of argon laseremitted with a wavelength of 456.1 nonometers and modulated in intensitywith eight-to-fourteen modulated audio signals. Because of thenoticeable absorptivity of the dye sensitizer in response to the laserbeam of the particular wavelength, the dye sensitizer is caused tosublime by the heat generated in the light-absorptive coating 4 at thosespots of the coating 4 at which the laser beam is focused. In thepresence of the cellulose nitrate admixed to the dye sensitizer,furthermore, the cellulose nitrate is also caused to sublime by reasonof the self-oxidation effect thereof. The sublimation of the cellulosenitrate promotes the sublimation of the dye sensitizer so that the layerof the cellulose nitrate and the dye sensitizer is ultimately formedwith the spaced pits 5 as shown in FIG. 1. Those areas of thelight-absorptive coating 4 where the laser beam has not been focussedremain undisturbed and intact. Since the ablation of thelight-absorptive coating 4 can not be caused unless the beam of lighthaving the predetermined wavelength in response to which the dyesensitizer in the light-absorptive coating 4 exhibits a noticeable lightabsorptivity is incident on the light-absorptive coating 4 with anenergy level higher than a predetermined threshold value, the laminarstructure including the light-absorptive coating 4 can be processedand/or treated in the light not only during the mastering step but alsoduring other steps of the disc forming process.

The signals recorded on the disc blank produced in this manner can beread out by irradiating the blank with a beam of helium-neon laser ofthe 0.3 milliwatt power and 632.8 nanometer wavelength. The theoreticalprinciple accounting for the fact that the signals can thus bereproduced in this fashion is not exactly known at this stage of theresearch and development. Here, suffice it to say that a principle isnot such that, since the dye sensitizer used exhibits practically nolight absorptivity in response to the wavelength of the helium-neonlaser beam, the intensity of the light reflected from thelight-absorptive coating 4 increases at least at the pits 5 in thelight-absorptive coating 4 with a consequent decrease in the intensityof the light reflected from the coating 4 over the remaining undisturbedareas of the coating 4. (Such a principle may be applied to reproductionof the signals provided an argon laser beam is used as a signal pick-upmedium.) The principle accounting for the fact that the signals can bereproduced by a helium-neon laser beam may probably be that theintensity of the light reflected from the light-absorptive coating 4decreases at the pits 5 due to the diffraction or interference of lighttaking place at the pits 5 and increases over the remaining undisturbedareas due to the presence of the light-reflective coating 3 underlyingthe light-absorptive coating 4. At all events, it is of importance forpractical purposes that the signals recorded on the recording disc 1 canbe reproduced by the use of a helium-neon laser beam as a signal pick-upmedium. While a signal reproducing apparatus using an argon laser beamrequires incorporation of an extra cooling equipment into the apparatusand is for this reason practically limited to industrial use, anapparatus using a helium-neon laser beam is applicable to home andprivate use (and has already been put into market) since the helium-neonlaser beam can be used as a signal pick-up medium without havingrecourse to the incorporation of such an equipment in the signalreproducing apparatus. This will mean that an information recording discaccording to the present invention can be played back by the use of asemiconductor laser beam having a larger wavelength.

Experiments were conducted with an information recording disc 1 having alight-absorptive coating 4 on which signals were recorded with use ofargon laser beams of different powers. The light-absorptive coating 4was formed of cellulose nitrate and a dye sensitizer which wereproportioned in the ratio of 86.5:13.5 on a weight basis. The recordingdisc 1 thus prepared was played back in a signal reproducing apparatususing a helium-neon laser beam as a signal pick-up medium. Tests weremade for changes in the levels of reproducing radio-frequency (RF)signals, changes in the locations of oscillographic eye patterns andchange in the average signal reproduction error factors per 30 secondsduring the periods of 10 to about 50 minutes. The following table showsthe results of these tests.

    ______________________________________                                        Recording RF Signal               Average                                     Power     Levels       Eye Pattern                                                                              Error                                       (mW)      (mVpp)       Locations  Factors                                     ______________________________________                                        2.0       190          Center     4 × 10.sup.-2                         2.5       300-400      Center     7 × 10.sup.-5                         2.8       460          Slightly   4 × 10.sup.-5                                                below Center                                           3.0       550          Slightly   4 × 10.sup.-5                                                below Center                                           3.5       650          Bottom     8 × 10.sup.-4                         4.0       685          Bottom     2 × 10.sup.-3                         4.5       690          Bottom     6 × 10.sup.-3                         ______________________________________                                    

When, on the other hand, an information recording disc having alight-absorptive coating of photoresist developed and formed withinformation tracks of pits and coated with a light-reflective coating ofmetal is played back, it was found that the radio-frequency signals wereon the levels of about 1000 to 1300 mVpp, the oscillographic eyepatterns were located at the center or slightly below the center, andthe average signal reproduction error factor was on the order of 10⁻⁴.It is thus understood that, when the power of the recording laser beamis selected at a suitable value (which ranges from 2.8 to 3.0milliwatts) in recording signals on an information recording discaccording to the present invention, the average error factor can beimproved about ten times the error factor in reproducing signals from aconventional disc using a light-absorptive coating of photoresist. Thismeans that signals are recorded on a recording disc according to thepresent invention with an accuracy which is increased about ten timesthe accuracy of recording of the signals on a disc using alight-absorptive coating of photoresist. In the case of an informationrecording disc produced in accordance with the present invention,furthermore, it has been ascertained that reproducing radio-frequencysignals of the levels of about 460 milliwatts to about 550 milliwattscan be obtained when the signals are reproduced immediately after themastering step. For this reason and further because of the fact that theoscillographic eye patterns were located slightly below the center, notonly the signals can be read out with ease and can be monitoredimmediately after the mastering step but production of chemical residuacan be avoided during formation of the pits in the light-absorptivecoating of the disc. The disc per se can therefore be put to use as afinal record with a practically acceptable signal-to-noise ratio.

If desired, the information recording disc 1 produced in accordance withthe present invention as hereinbefore described may be used to produce astamper. For this purpose, the light-absorptive coating 4 of therecording disc 1 is coated with an electrically conductive film byapplication of metal such as silver to the surface of the coating 4 by,for example, a vacuum evaporation, sputtering or non-electrolyticprocess. A layer of a release compound is formed on the conductive filmand is then coated with metal such as nickel applied to the layer of therelease compound by an electrotyping process. A stamper is thus obtainedby releasing the outermost layer of metal from the recording disc 1. Themetal disc prepared in this fashion may be utilized not directly as astamper but as a master matrix to produce a mother matrix from which asub-master matrix to produce a stamper is to be produced. The stamperthus produced may be used to produce replicas for use in a playbackapparatus of the optical, static capacitance or piezoelectric type byselection of the material to form the replicas.

When the laminar structure of the substrate 2 and the light-absorptiveand light-reflective coatings 3 and 4 is found unacceptable bymonitoring the recorded signals during or immediately after themastering step, the process is brought to an end promptly. In thisinstance, a new light-absorptive coating can be formed on the substrate2 after removal of the light-absorptive coating 4 from the substrate 2by application of a suitable organic solvent to the coating 4. Thus, aninformation recording disc provided in accordance with the presentinvention is more economical than a conventional information recordingdisc using a light-absorptive film of photoresist in the case of whichthe substrate cleared of the photoresist coating must be polished toremedy the roughness of the surface before a new coating of photoresistis formed on the substrate. The inorganic solvent to be used forremoving the light-absorptive coating 4 from the substrate 2 as abovedescribed may be xylene but is preferably acetone for its high dryingrate.

It will be apparent that, if the monitoring of the signals recorded onthe light-absorptive coating 4 is not necessitated, the light-absorptivecoating 4 may be formed on the substrate 2 directly, viz., withoutforming the light-reflective coating 3 between the substrate 2 andlight-absorptive coating 4. It will also be apparent that the signals tobe recorded on an information recording disc provided in accordance withthe present invention are not limited to audio and/or video signals andthat the disc per se can be used not only as a video disc and apulse-code-modulated audio disc but also as any other type ofinformation recording disc.

What is claimed is:
 1. An information recording disc, comprising:agenerally circular substrate, and a light-absorptive coating secured tosaid substrate, said light-absorptive coating being a mixture ofcellulose nitrate having a degree of nitration higher than about 13% anda dye sensitizer, said dye sensitizer present in an amount sufficientfor said coating to exhibit a noticeable light absorptivity, saidcellulose nitrate being prepared from a starting cellulose containing atleast about 97% alpha-cellulose.
 2. An information recording disc,comprising:a generally circular substrate, a light-reflective coatingformed on said substrate, and a light-absorptive coating formed on thesurface of the light-reflective coating said light-absorptive coatingbeing a mixture of cellulose nitrate having a degree of nitration higherthan about 13%, and a dye sensitizer, said dye sensitizer present in anamount sufficient for said coating to exhibit a noticeable lightabsorptivity, said cellulose nitrate being prepared from a startingcellulose containing at least about 97% alpha-cellulose.
 3. Aninformation recording disc as set forth in claim 1 or 2, in which saidcellulose nitrate has a number average degree of polymerization withinthe range of from about 80 to about
 95. 4. An information recording discas set forth in claim 1 or 2, in which said cellulose nitrate has adegree of nitration higher than about 13 per cent and a number averagedegree of polymerization within the range of from about 80 to about 95.5. An information recording disc as set forth in claim 1 or 2, in whichsaid dye sensitizer is N-ethyl-N-oxyethylaniline
 6. An informationrecording disc as set forth in claim 1 or 2, in which the ratio inweight between said cellulose nitrate and said dye sensitizer is from86.5:13.5 to 80:20.